Optical pickup device and optical disk device having a diffraction grating

ABSTRACT

An optical pickup device is adapted to read information signals from optical recording media of two different types such as a “DVD” and a “CD” for which two light beams with different wavelengths are used and comprises a photodetector having a single light receiving section that can be shared by optical recording media of two different types. Both the light beam reflected from the signal recording surface of the “DVD”  106   a  and the light beam reflected from the signal recording surface of the “CD”  106   b  are diffracted by the diffraction element  6  and focussed to a same spot on the light receiving surface of the photodiode of a photodetector  7.

RELATED APPLICATION

This application is a divisional application of application Ser. No.09/801,343, filed on Mar. 8, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical pickup device for writinginformation signals onto and reading information signals from an opticalrecording medium and also to an optical disc device provided with suchan optical pickup device and adapted to record and reproduce informationsignals, using an optical disc as optical recording medium.

2. Related Background Art

Optical discs are known as optical recording medium and various opticalpickup devices for reading information signals from an optical recordingmedium have been proposed to date.

Popular optical discs include “Compact Discs” (tradename, to be referredto as “CD” hereinafter) and “Digital Versatile Discs (tradename, to bereferred to as “DVD” hereinafter) that can store information signalsmuch more densely than CDs. Meanwhile, optical pickup devices adapted toread information signals from both “CDs” and “DVDs” are also known.

Referring to FIG. 1 of the accompanying drawings, an optical pickupdevice adapted to read information signals from both “CDs” and “DVDs”comprises a laser diode (LD) 101 operating as light source. The laserbeam emitted from the laser diode 101 is typically red light (e.g.,having a wavelength of 635 nm) and fed to a beam splitter 103 by way ofa diffraction grating 102. The diffraction grating 102 is used togenerate a sub-beam for detecting a tracking error signal. The beamsplitter 103 is a plate having a pair of parallel surface planes thatare inclined by 45 relative to the optical axis of the laser beam comingfrom the laser diode 101. The laser beam emitted from the laser diode101 is reflected and deflected by 90 by the corresponding surface planeof the beam splitter 103 before it is collimated by a collimator lens104 and enters an objective lens 105. The objective lens 105 focuses theincident laser beam on the signal recording surface of a “DVD” 106 a ora “CD” 106 b.

The laser beam focused on the signal recording surface of either the“DVD” 106 a or the “CD” 106 b is modulated according to the informationsignal recorded on the “DVD” 106 a of the “CD” 106 b, whicheverappropriate″ and reflected so that it returns to the objective lens 105as reflected laser beam. The reflected laser beam then gets to the beamsplitter 103 by way of the collimator lens 104. As the reflected laserbeam is transmitted through the beam splitter 103, it gives rise toastigmatism and is subsequently focused on the light receiving surfaceof a photodetector (PD) 107. A focusing error signal, if any, can bedetected on the basis of the astigmatism generated as a result of beingtransmitted through the beam splitter 103.

If the laser beam emitted from the light source is red light having asingle wavelength, it cannot read any information signal from a “CD-R”that uses a coloring matter for the signal recording layer. This isbecause the reflectivity of the signal recording surface of a “CD-R” isvery low relative to red light.

In view of this fact, there has been proposed an optical pickup devicecomprising a pair of light sources that are adapted to emit beams withdifferent wavelengths as shown FIG. 2 so that it may read informationsignals not only from “CDs” and “DVDs” but also from “CD-Rs” whoseoperation is highly dependent on the wavelength of light to be used withit.

With such an optical pickup device, a beam of red light (e.g., having awavelength of 635 nm) is emitted from a laser diode 101 operating as thefirst light source and an infrared beam (e.g., having a wavelength of780 nm) is emitted from a laser chip comprising a light receiving/lightemitting composite element 109 and operating as the second light source.

The laser beam emitted from the laser diode 101 is fed to a beamsplitter 103. The beam splitter 103 is a plate having a pair of parallelsurface planes that are inclined by 45 relative to the optical axis ofthe laser beam coming from the laser diode 101. The laser beam emittedfrom the laser diode 101 is reflected and deflected by 90 by thecorresponding surface plane of the beam splitter 103 before it iscollimated by a dichroic beam splitter 108 and a collimator lens 104,and enters an objective lens 105. The objective lens 105 focuses theincident laser beam on the signal recording surface of a “DVD” 106 a.

On the other hand, the laser beam emitted from the laser chip comprisingthe light receiving/light emitting composite element 109 is fed to adichroic beam splitter 108. The dichroic beam splitter 108 has areflection plane that is inclined by 45 relative to the optical axis ofthe laser beam coming from the laser chip comprising the lightreceiving/light emitting composite element 109. The laser beam emittedfrom the laser chip is reflected and deflected by 90 by the reflectionplane. The laser beam emitted from the laser chip comprising the lightreceiving/light emitting composite element 109 and the laser beamemitted from the laser diode 101 are made to have a same and identicaloptical axis. The laser beam emitted from the laser chip comprising thelight receiving/light emitting composite element 109 is then collimatedby the collimator lens 104 and enters the objective lens 105. Theobjective lens 105 focuses the incident laser beam on the signalrecording surface of a “CD” 106 b.

The laser beam focused on the signal recording surface of either the“DVD” 106 a or the “CD” 106 b is then reflected by the signal recordingsurface thereof so that it returns to the objective lens 105 asreflected laser beam. The reflected laser beam then gets to thecollimator lens 104 and the dichroic beam splitter 108. Since thedichroic beam splitter 108 transmits red light but reflects infraredbeams, the optical path of the red beam and that of the infrared laserbeam are separated from each other there.

The red beam transmitted through the dichroic beam splitter 108 thengets to the beam splitter 103 and, as it is transmitted through the beamsplitter 103, it gives rise to astigmatism and is subsequently focusedon the light receiving surface of a photodetector (PD) 107.

On the other hand, the infrared beam reflected by the reflection planeof the dichroic beam splitter 108 is focused on the light receivingsurface of the photodetector of the light receiving/light emittingcomposite element 109.

Meanwhile, as a result of the advancement of semiconductor technologiesin recent years, it has become possible to mount a pair of laser chipson a same semiconductor substrate as shown in FIG. 3 by using theso-called monolithic technology. More specifically, the light emittingspots 111 a, 111 b of a pair of laser chips can be arrangedtransversally side by side with a gap of only 80 μm to 200 μm separatingthem.

A light receiving/light emitting composite element 110 comprising a pairof laser chips 111 a, 111 b can be formed by arranging a photodetector112 on a semiconductor substrate 114 in addition to the laser chips 111a, 111 b. The light receiving/light emitting composite element 110 isadditionally provided with a prism 113 arranged on the photodetector 113and having its sloped plane faced to the laser chips 111 a, 111 b.

With the light receiving/light emitting composite element 110, the laserbeams emitted from the laser chips 111 a, 111 b are reflected by thesloped plane of the prism 113 and directed to the outside of the lightreceiving/light emitting composite element 110. Then, they are reflectedback to the light receiving/light emitting composite element 110 by thecorresponding optical recording medium to enter the prism 113 and becomedetected by the photodetector.

Referring now to FIG. 4, with the optical pickup device that is adaptedto read information signals from both a “CD” and a “DVD” by using amonolithic laser diode, the laser beams emitted from the laser chips 111a, 111 b of the light receiving/light emitting composite element 110 arefed to the objective lens 105 by way of the collimator lens 104 so as tobe focused on the signal recording surface of the “DVD” 106 a or the“CD” 106 b by the objective lens 105. Then, the laser beam reflected bythe signal recording surface of the “DVD” 106 a or the “CD” 106 b,whichever appropriate, is fed back to the light receiving/light emittingcomposite element 110 and received by the photodetector of the lightreceiving/light emitting composite element 110.

In the light receiving/light emitting composite element 110, the pair oflaser chips 111 a, 111 b that are used respectively for a “DVD” and a“CD” are separated from each other by a gap of about 120 μm. Then, thetwo laser beams emitted from the respective laser chips 111 a, 111 b aremade to strike the optical recording medium, keeping the distance of 120μm separating their optical axes from each other, so as to be reflectedby the signal recording surface of the optical recording medium and fedback to the photodetector 112 of the light receiving/light emittingcomposite element 110.

The photodetector 112 has a first light receiving surface for receivingthe laser beam to be used for a “DVD” that is reflected by the signalrecording surface of a “DVD” and a second light receiving surface forreceiving the laser beam to be used for a “CD” that is reflected by thesignal recording surface of a “CD”. The first and second light receivingsurfaces are separated by a gap of about 120 μm, which is same as thegap separating the laser chips 111 a, 111 b.

The above described optical system is so regulated that the laser beamto be used for a “DVD” occupies the center of the optical axis of theoptical system and hence the laser beam to be used for a “CD” isdisplaced from the optical axis by the distance same as the distanceseparating the light emitting spots of the laser diode, or 120 μm, inview of the fact that the operation of reading information signals froma “DVD” is more difficult than that of reading information signals froma “CD”. The use of a hologram element has been proposed to correct thedisplacement of the laser beam due to the displaced light emitting spotthereof.

The use of a monolithic laser diode for an optical pickup deviceprovides advantages including a reduced number of components,down-sizing and easier regulating operations during the manufacturingprocess.

It is also possible to form an optical system, using a monolithic diodein a discrete way as shown in FIG. 5. The laser diode 101 a of theoptical pickup device of FIG. 5 comprises first and second laser chips111 a, 111 b as shown in FIG. 6. The laser beams emitted from the laserdiode 101 a typically include a red laser beam and an infrared laserbeam that are fed to a beam splitter 103 by way of a diffraction grating102. The diffraction grating 102 is used to generate a sub-beam fordetecting a tracking error signal. The beam splitter 103 is a platehaving a pair of parallel surface planes that are inclined by 45relative to the optical axis of the laser beam coming from the laserdiode 101 a. The laser beam emitted from the laser diode 101 a isreflected and deflected by 90 by the corresponding surface plane of thebeam splitter 103 before it is collimated by a collimator lens 104 andenters an objective lens 105. The objective lens 105 focuses theincident laser beam on the signal recording surface of a “DVD” 106 a ora “CD” 106 b.

The laser beam focused on the signal recording surface of either the“DVD” 106 a or the “CD” 106 b is modulated according to the informationsignal recorded on the “DVD” 106 a or the “CD” 106 b, whicheverappropriate and reflected so that it returns to the objective lens 105as reflected laser beam. The reflected laser beam then gets to the beamsplitter 103 by way of the collimator lens 104. As the reflected laserbeam is transmitted through the beam splitter 103, it gives rise toastigmatism and is subsequently focused on the light receiving surfaceof a photodetector (PD) 107. A focusing error signal, if any, can bedetected on the basis of the astigmatism generated as a result of beingtransmitted through the beam splitter 103.

With any of the above described optical pickup devices comprising a pairof light emitting spots, the two light emitting spots are separated fromeach other at least by a distance of about 80 m in view of the spatialrestrictions imposed on it. Thus, in the case of a confocal opticalsystem, two focal points are formed on the respective light receivingsurfaces of the photodetector and separated from each other by adistance of about 80 m. Therefore, a pair of light receiving surfacesare arranged in the photodetector and separated from each other by atleast about 80 m in order to receive the two laser beams that arefocused to the respective focal points.

A discrete optical system is advantageous relative to an integratedoptical system as shown in FIGS. 3 and 4 because it involves lessdiffracted light that is unnecessary to the system and it can bemanufactured more easily.

With any of the above described optical pickup devices comprising a pairof light emitting spots, the two light emitting spots are separated fromeach other at least by a distance of about 80 μm in view of the spatialrestrictions imposed on it. Thus, in the case of a confocal opticalsystem, two focal points are formed on the respective light receivingsurfaces of the photodetector and separated from each other by adistance of about 80 μm. Therefore, a pair of light receiving surfacesare arranged in the photodetector and separated from each other by atleast about 80 μm in order to receive the two laser beams that arefocussed to the respective focal points.

If the optical disc to be used with such an optical pickup device isdriven to rotate at high speed in order to read information signalstherefrom as in the case of a “CD-ROM”, it is necessary to arrange apair of I-V amplifiers near the respective light receiving surfaces onthe semiconductor substrate of the photodetector. However, with thephotodetector of any of the above described optical pickup devices, itis highly difficult to arrange such a pair of I-V amplifiers on thesemiconductor substrate because the light receiving surfaces arearranged so tightly relative to each other with such a narrow gapseparating them.

This problem may be dissolved by separating the two light emitting spotsby a large gap. However, with a pair of light emitting spots that areseparated from each other by a large distance, one of the laser beamswill inevitably be displaced from the optical axis of the optical systemalso by a large distance to consequently degrade the optical performanceof the optical system.

BRIEF SUMMARY OF THE INVENTION

In view of the above identified circumstances, it is therefore an objectof the present invention to provide an optical pickup device adapted toread information signals from optical recording media of two differenttypes such as a “DVD” and a “CD” for which two light beams withdifferent wavelengths are used and comprising a photodetector having asingle light receiving section that can be shared by optical recordingmedia of two different types.

Another object of the present invention is to provide an optical discdevice comprising such an optical pickup device.

According to the invention, the above objects are achieved by providingan optical pickup device comprising:

a first light source for emitting a first light beam having a firstwavelength;

a second light source for emitting a second light beam having a secondwavelength different from the first wavelength;

an objective lens for focusing said light beam or said second light beamto the signal recording surface of an optical of a first type matchingto the first wavelength or that of an optical of a second type matchingto the second wavelength, whichever appropriate;

a photodetector for detecting the light beam focused on the signalrecording surface of the an optical of the first type or that of the anoptical of the second type, whichever appropriate, by the objective lensand reflected by the signal recording surface; and

a diffraction element arranged on the light path from the light sourcesto the photodetector by way of the two pieces of optical recordingmedium;

at least either the first light beam adapted to be used for readinginformation signals from the signal recording surface of the an opticalof the first type and reflected by the reflecting surface or the secondlight beam adapted to be used for reading information signals from thesignal recording surface of the an optical of the second type andreflected by the reflecting surface being diffracted by the diffractionelement, the first reflected light beam and the second reflected lightbeam being focused to a same spot on the light receiving surface of thephotodetector.

When both the first reflected light beam and the second reflected lightbeam are diffracted by the diffraction element, the diffraction elementcan be made to show a large angle of diffraction to consequently reducethe influence of any diffracted light that is unnecessary to thephotodetector.

When the diffraction element is arranged on the light path between thetwo pieces of optical recording medium and the photodetector, it can beused efficiently for the light beam particularly when the light sourceis a laser diode regardless if the light beam is emitted from the firstlight source or from the second light source.

When the diffraction element is arranged on the light path between thelight sources and the two pieces of optical recording medium, theoptical axis of the first light beam striking the corresponding opticalrecording medium and that of the second light beam striking thecorresponding optical recording medium can be made to accurately agreewith each other.

The influence of the ambient temperature on the optical performance ofthe optical pickup device can be minimized by arranging a pair ofdiffraction gratings on the opposite surfaces of a single piece ofmedium to form the diffraction element.

On the other hand, when the diffraction element is formed by using twopieces of medium, each carrying a diffraction grating on one of theopposite surfaces thereof, the influence of the ambient temperature onthe optical performance of the optical pickup device can also beminimized and, additionally, the position of the light spot on thephotodetector can be regulated by moving and regulating the diffractionelement.

According to the invention, there is also provided an optical discdevice comprising an optical pickup device according to the inventionand a rotary operating mechanism for driving the optical disc, or theoptical recording medium, to rotate and operate it, the optical pickupdevice being arranged opposite to the signal recording surface of theoptical disc to be driven to rotate and operated by the rotary operatingmechanism.

As pointed out above, in an optical pickup device according to theinvention, either the first light beam adapted to be used for readinginformation signals from the signal recording surface of the recordingmedium of the first type and reflected by the reflecting surface or thesecond light beam adapted to be used for reading information signalsfrom the signal recording surface of the optical recording medium of thesecond type and reflected by the reflecting surface is diffracted by thediffraction element and the first reflected light beam and the secondreflected light beam are focused to a same spot on the light receivingsurface of the photodetector.

When both the first reflected light beam and the second reflected lightbeam are diffracted by the diffraction element, the diffraction elementcan be made to show a large angle of diffraction to consequently reducethe influence of any diffracted light that is unnecessary to thephotodetector.

When the diffraction element is arranged on the light path between thetwo pieces of optical recording medium and the photodetector, it can beused efficiently for the light beam particularly when the light sourceis a laser diode regardless if the light beam is emitted from the firstlight source or from the second light source.

When the diffraction element is arranged On the light path between thelight sources and the two pieces of optical recording medium, theoptical axis of the first light beam striking the corresponding opticalrecording medium and that of the second light beam striking thecorresponding optical recording medium can be made to accurately agreewith each other.

The influence of the ambient temperature on the optical performance ofthe optical pickup device can be minimized by arranging a pair ofdiffraction gratings on the opposite surfaces of a single piece ofmedium to form the diffraction element.

On the other hand, when the diffraction element is formed by using twopieces of medium, each carrying a diffraction grating on one of theopposite surfaces thereof, the influence of the ambient temperature onthe optical performance of the optical pickup device can also beminimized and, additionally, the position of the light spot on thephotodetector can be regulated by moving and regulating the diffractionelement.

Thus, according to the invention, there is provided an optical pickupdevice adapted to read information signals from optical recording mediaof two different types such as a “DVD” and a “CD” for which two lightbeams with different wavelengths are used and comprising a photodetectorhaving a single light receiving section that can be shared by opticalrecording media of two different types whereas maintaining a stableoperation.

According to the invention, there is also provided an optical discdevice comprising such an optical pickup device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic lateral view of the optical system of a knownoptical pickup device of the type using a single wavelength;

FIG. 2 is a schematic lateral view of the optical system of a knownoptical pickup device of the type comprising two light sources and usingtwo wavelengths;

FIG. 3 is a schematic perspective view of a light receiving/lightemitting composite element of the two-wavelength type used in a knownoptical pickup device;

FIG. 4 is a schematic lateral view of the optical system of a knownoptical pickup device comprising a light receiving/light emittingcomposite element of the two-wavelength type.

FIG. 5 is a schematic lateral view of the optical system of a knownoptical pickup device comprising a monolithic laser diode and using twowavelengths;

FIG. 6 is a schematic perspective view of the monolithic laser diode ofthe two-wavelength type used in a known optical pickup device;

FIG. 7 is an enlarged schematic plane view of a principal part of thelight receiving section of the photodetector of the known optical pickupdevice of FIG. 5;

FIG. 8 is a schematic lateral view of the optical system of anembodiment of optical pickup device according to the invention;

FIG. 9 is a schematic lateral view of the diffraction element of theembodiment of optical pickup device of FIG. 8, showing its profile;

FIG. 10 is an enlarged schematic lateral view of a principal part of thediffraction element of FIG. 9, showings its profile;

FIG. 11 is an enlarged schematic plan view of the light receivingsection of the photodetector of the optical pickup device of FIG. 8.

FIG. 12 is a schematic lateral view of the optical system of anotherembodiment of optical pickup device according to the invention;

FIG. 13 is a schematic lateral view of the diffraction element of theembodiment of optical pickup device of FIG. 12, showing its profile;

FIG. 14 is an enlarged schematic lateral view of a principal part of thediffraction element of FIG. 13, showings its profile;

FIG. 15 is a schematic lateral view of the optical system of stillanother embodiment of optical pickup device according to the invention;

FIG. 16 is a schematic lateral view of the diffraction element of theembodiment of optical pickup device of FIG. 15, showing its profile;

FIG. 17 is an enlarged schematic lateral view of a principal part of thediffraction element of FIG. 16, showings its profile;

FIG. 18 is a schematic lateral view of the optical system of stillanother embodiment of optical pickup device according to the invention,comprising a two-wavelength type monolithic laser diode and adiffraction element arranged on the backward light path;

FIG. 19 is a schematic lateral view of the diffraction element of theembodiment of optical pickup device of FIG. 18, showing its profile;

FIG. 20 is an enlarged schematic plane view of a principal part of thelight receiving section of the photodetector of the embodiment ofoptical pickup device of FIG. 18; and

FIG. 21 is a schematic lateral view of the optical system of stillanother embodiment of optical pickup device according to the invention,comprising a two-wavelength type monolithic laser diode and adiffraction element arranged on the forward light path.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in greater detail byreferring to the accompanying drawings that illustrate preferredembodiments of the invention.

FIG. 8 is a schematic lateral view of the optical system of the firstembodiment of optical pickup device according to the invention andadapted to use a so-called “DVD (Digital Versatile Disc)” (tradename) asoptical recording medium of the first type and a so-called “CD (CompactDisc)” as optical recording medium of the second type. Typically, the“DVD” is adapted to a wavelength of 635 nm, while the “CD” is adapted toa wavelength of 780 nm.

As shown in FIG. 8, the optical pickup device comprises an opticalsystem realized by discretely using a monolithic laser diode. The laserdiode of the optical pickup device has two laser chips, a first laserchip and a second laser chip, contained in a single package. The twolaser chips are separated from each other by a distance between 80 μmand 120 μm. The first laser chip is adapted to emit a first laser beamthat is a red laser beam (with a wavelength of 635 nm), whereas thesecond laser chip is adapted to emit a second laser beam that is aninfrared laser beam (with a wavelength of 780 nm).

The light beam emitted from the laser diode 1 gets to a beam splitter 3by way of a diffraction grating 2. The diffraction grating 2 is used togenerate a pair of sub-beams to be used for detecting a tracking errorsignal. The beam splitter 3 is a plate having a pair of parallel surfaceplanes that are inclined by 45 relative to the optical axis of the laserbeam coming from the laser diode 1. The laser beam emitted from thelaser diode 1 is reflected and deflected by 90 by the correspondingsurface plane of the beam splitter 3 before it is collimated by acollimator lens 4 and enters an objective lens 5. The objective lens 5focuses the incident laser beam on the signal recording surface of a“DVD” 106 a or a “CD” 106 b.

The laser beam focused on the signal recording surface of either the“DVD” 106 a or the “CD” 106 b is modulated according to the informationsignal recorded on the “DVD” 106 a of the “CD” 106 b, whicheverappropriate” and reflected so that it returns to the objective lens 5 asreflected laser beam.

The reflected laser beam then gets to the beam splitter 3 by way of thecollimator lens 4. As the reflected laser beam is transmitted throughthe beam splitter 3, it gives rise to astigmatism and is subsequentlytransmitted through a diffraction element 6. As shown in FIG. 9, thediffraction element 6 is realized by forming a diffraction gratingpattern 6 a on one of the opposite surfaces of a plate of a mediumhaving a pair of parallel surface planes. As shown in FIG. 10, thediffraction grating pattern 6 a is that of a blazed diffraction gratingdesigned to enhance the diffraction efficiency of a specific degree. Forinstance, if the diffracted light beam of the 1st degree is used foreach of the reflected laser beams, the diffraction grating pattern ofthe diffraction element 6 is made to show a depth somewhere between thedepth that maximizes the primary diffraction efficiency of the firstreflected light beam and the depth that maximizes the primarydiffraction efficiency of the second reflected light beam.

Both the diffracted light beam of the 1st degree of the first reflectedlight beam produced by the diffraction element 6 and that of the secondreflected light beam produced by the diffraction element 6 are thenfocused to the focal point on the light receiving section 7 a of thephotodetector 7. With this optical pickup device, it should bereiterated that both the diffracted light beam of the 1st degree of thefirst reflected light beam and that of the second reflected light beamare focused to a same focal point on a same light receiving section 7 a.Both the diffracted light beam of the 1st degree of the first reflectedlight beam and that of the second reflected light beam are focused to asame focal point on a same light receiving section 7 a because thedistance separating the focal points of the two light beams thatcorresponds to the distance separating the two light emitting spots ofthe laser diode 1 is offset by the difference in the diffraction angleof the two reflected light beams.

For a diffraction grating, if the cycle of the grating is a, thewavelength of the incident laser beam is e and the degree of diffractionis n, the diffraction angle è of the diffracted light beam of the n-thdegree is expressed by formula below.sin è=në/a

The photodetector 7 is adapted to detect a focusing error signal, ifany, on the basis of the astigmatism produced when the light beam istransmitted through the beam splitter 3. The pair of sub-beams generatedby the diffraction grating 2 also gets to the photodetector 7 by way ofthe above described light path. A focusing error signal can be detectedby the photodetector 7 by detecting the reflected light beams of thesub-beams. More specifically, as shown in FIG. 11, the photodetector 7comprises a main light receiving section 7 a and a pair of auxiliarylight receiving sections 7 b, 7 c for receiving the reflected lightbeams of the sub-beams. The main light receiving section 7 a is dividedinto four light receiving areas that are arranged radially as viewedfrom the center thereof so that the photodetection output of each of thelight receiving areas can be obtained independently. Then, a focusingerror signal can be obtained by means of a so-called astigmatism methodof performing arithmetic operations using the photodetection outputs ofthe light receiving areas. On the other hand, each of the pairedauxiliary light receiving sections 7 b, 7 c has a single light receivingarea and they can obtain their respective photodetection outputsindependently. A tracking error signal, if any, can be detected by thephotodetector 7 by means of a so-called 3-beam method of performingarithmetic operations using the photodetection outputs of the auxiliarylight receiving sections 7 b, 7 c.

With this optical pickup device, a large diffraction angle can be usedfor the diffraction element 6 to minimize the influence of anydiffracted light that is unnecessary to the photodetector because boththe diffracted light beam of the 1st degree the first reflected lightbeam and that of the second reflected light beam of the diffractionelement 6 are used for reading information signals.

With the above described optical pickup device, the spots of thereflected light beams formed on the light receiving sections of thephotodetector can be shifted as a result of a change in the diffractionangle of the diffraction element due to the change in the wavelength ofany of the light beams emitted from the laser diode that is byfluctuations in the ambient temperature. However, such a shift of thespots of light in the photodetector can be corrected by using a mediumwhose refractive index changes as a function of temperature such as aplastic material for the beam splitter so that the shift of the spots oflight caused by the change in the refractive index may be offset by theshift of the spots of light caused by the change in the diffractionangle of the diffraction element.

Additionally, with the above described optical pickup device, the spotsof the reflected light beams in the photodetector can be shifted in adirection perpendicular to the optical axis by shifting and regulatingthe position of the diffraction element. Therefore, any positionaldisplacement of the spots of light due to the dimensional errors of anyof the optical elements of the device can be corrected by shifting andregulating the position of the diffraction element. The sensitivity ofthe diffraction element to such a regulating operation rises as thediffraction angle of the diffraction element is increased.

FIG. 12 is a schematic lateral view of the optical system of anotherembodiment of optical pickup device according to the invention, whereinthe diffraction element 8 arranged on the optical path between the beamsplitter 3 and the photodetector 7 is prepared by forming a diffractiongrating on each of a pair of oppositely disposed surfaces of a singlepiece of medium as shown in FIG. 13. Note that, in FIG. 14, thediffraction patterns arranged on the opposite surfaces of the mediumthat is a plate having a pair of parallel surface planes are mirrorimages relative to each other.

With this arrangement, it is not necessary for the first reflected lightbeam and the second reflected light beam that the laser beams getting tothe light receiving sections of the photodetector 7 are diffracted laserbeams. For example, it may be so arranged that the light beam of the0-th degree (or the light beam that is not diffracted) of thediffraction element 8 is detected by the photodetector 7 for the firstreflected light beam and the light beam of the 1st order of thediffraction element 8 is detected by the photodetector for the secondreflected light beam.

When such a diffraction element 8 is used, the influence of the ambienttemperature on the performance of the diffraction element 8 can beminimized. More specifically, since changes occur symmetrically in theperformance of the diffraction patterns due to the fluctuations of theambient temperature on the opposite surfaces of the medium of thediffraction element 8, the change in the diffraction efficiency relativeto one of the reflected laser beams is offset by the change in thediffraction efficiency relative to the other reflected light beam sothat the light beams transmitted through the diffraction element 8 arenot influenced by such changes.

Additionally, the changes in the diffraction angle due to the changes inthe oscillation wavelengths of the laser diode are offset by each otherdue to the diffraction patterns on the opposite surfaces of thediffraction element 8, the spots of the reflected light beams on thelight receiving sections of the photodetector 7 are not shifted if theoscillation frequencies of the laser diode are made to vary due to thefluctuations of the ambient temperature.

With the above arrangement, since any shift of the optical axis of eachof the reflected light beams is corrected only during the light beam istransmitted between the two diffraction patterns, or through the mediumof the diffraction element 8, the diffraction angle is large if comparedwith the arrangement of FIG. 8 where the shift of the optical axis iscorrected over the entire light path between the diffraction element 8and the photodetector.

When the diffraction element 8 is arranged on the backward light pathfrom the optical discs 106 a, 106 b to the photodetector 7 as shown inFIG. 12, both the light beam emitted from the first light source and thelight beam emitted from the second light source can be used efficientlyif the light sources are those of a laser diode and the dispersion angleof each of the light beams therefrom is limited.

Note that the diffraction element 8 may alternatively be arranged on theforward light path from the light sources to the optical discs. If suchis the case, the optical axis of the first light beam and that of thesecond light beam respectively striking the optical discs 106 a, 106 bcan be made to accurately agree with each other.

Still alternatively, it may be so arranged that both of the light beamsto be detected by the photodetector 7 for the first reflected light beamand the second reflected light beam are diffracted by the diffractionelement 8. More specifically, for examples, it may be so arranged thatthe diffracted light beam of the 1st degree from the diffraction elementfor the first reflected light beam is detected by the photodetector 7and the diffracted light beam of the 2nd degree from the diffractionelement 8 for the second reflected light beam is detected by thephotodetector 7. With this arrangement, the diffraction angle of thediffraction element becomes large to reduce the influence of anydiffracted light that is unnecessary to the photodetector.

FIG. 15 is a schematic lateral view the optical system of still anotherembodiment of optical pickup device according to the invention, whereinthe a pair of diffraction elements 9, 10 are arranged on the light pathfrom the light sources to the photodetector and each of the diffractionelements 9, 10 carries thereon a diffraction pattern on one of the pairof parallel surface planes as shown in FIG. 16. Note that, as shown inFIG. 17, the diffraction patterns arranged on the correspondingrespective surfaces of the plates, each having a pair of parallelsurface planes, are mirror images relative to each other.

With this arrangement, the light beams to be detected by thephotodetector 7 for the first reflected light beam and the secondreflected light beam do not need to be diffracted by the respectivediffraction elements 9, 10. More specifically, for examples, it may beso arranged that the diffracted light beam of the 0-th degree (or thelight beam that is not diffracted) from the diffraction elements 9, 10for the first reflected light beam is detected by the photodetector 7and the diffracted light beam of the 1st degree from the diffractionelements 9, 10 for the second reflected light beam is detected by thephotodetector 7.

With this arrangement or providing a pair of diffraction elements 9, 10,the influence of the fluctuations of the ambient temperature can beminimized. Since the change in the diffraction pattern due to thefluctuations of the ambient temperature occurs both in the medium of thediffraction element 9 and that of the diffraction element 10 in asimilar manner, the effect of diffraction of one of the reflected lightbeam offsets that of the other reflected light beam so that consequentlythe light beams transmitted through the diffraction elements 9, 10 arenot affected.

Additionally, the change in the diffraction angle due to the change inthe oscillation wavelength of each of the laser chips of the laser diodeis offset by the diffraction patterns of the diffraction elements 9, 10so that consequently the spots of reflected light on the light receivingsections of the photodetector 7 are not shifted if the oscillationfrequencies of the laser diode are made to vary by the fluctuations inthe ambient temperature. While the displacement of the optical pathbetween the diffraction patterns can not be corrected, it is about 1 μmif the wavelength of either of the light beams changes by 7 nm and hencenegligible.

Additionally, in the above described embodiment of optical pickupdevice, the spots of the reflected light beams on the light receivingsections of the photodetector 7 can be positionally regulated byshifting the diffraction elements 9, 10. Since the medium of thediffraction grating 9 and that of the diffraction grating 10 can beshifted independently, it is easy to shift and regulate the spots of thereflected light beams on the light receiving sections of thephotodetector 7. The diffraction angle of either of the diffractionelements 9, 10 can be raised without limitations if it is so arrangedthat the distance between the diffraction elements 9, 10 can beregulated along the direction of the optical axis.

In the above described embodiment of optical pickup device, the opticalaxis of the first light beam and that of the second light beam strikingthe respectively optical discs 106 a, 106 b can be made to accuratelyagree with each other, if the diffraction elements 9, 10 are arranged onthe forward light path from the light sources to the optical discs.Additionally, when the diffraction elements 9, 10 are arranged on theforward light path, any unnecessary diffracted light can be practicallyblocked from entering the photodetector even if the diffraction angle ofeither of the diffraction elements 9, 10 is small.

The diffraction elements 9, 10 may alternatively be arranged on thebackward light path from the optical discs 106 a, 106 b to thephotodetector 7. With such an arrangement, the light beam emitted fromeither of the first and second light source can be utilized efficientlyeven if the light sources are those of a laser diode and the dispersionangle of each of the light beams therefrom is limited.

Still alternatively, it may be so arranged that both of the light beamsto be detected by the photodetector 7 for the first reflected light beamand the second reflected light beam are diffracted by the diffractionelements 9, 10. More specifically, for examples, it may be so arrangedthat the diffracted light beam of the 1st degree from the diffractionelements 9, 10 for the first reflected light beam is detected by thephotodetector 7 and the diffracted light beam of the 2nd degree from thediffraction elements 9, 10 for the second reflected light beam isdetected by the photodetector 7. With this arrangement, the diffractionangle of the diffraction element becomes large to reduce the influenceof any diffracted light that is unnecessary to the photodetector.

FIG. 18 is a schematic lateral view of the optical system of stillanother embodiment of optical pickup device according to the invention,comprising a two-wavelength type monolithic laser diode and adiffraction element 6 arranged on the backward light path so that thelight beam for a “DVD” and the light beam for a “CD” may be focused on asame spot on a light receiving surface by using the diffracted lightbeams from the diffraction element 6. As shown in FIG. 19, thediffraction element 6 is a blazed diffraction grating adapted to raisethe intensity of the diffracted laser beam of a specific degree.

As shown in FIG. 20 illustrating an enlarged schematic plane view of aprincipal part of the light receiving section of the photodetector 7 ofthe embodiment of optical pickup device of FIG. 18, the light receivingsection 7 a of the photodetector 7 for receiving the reflected lightbeam to be used for a “DVD” is adapted to also receive the reflectedlight beam to be used for a “CD”.

With the optical system of this embodiment of optical pickup device, thediffracted light beam of the 0-th degree obtained from the diffractionelement 6 for the light beam emitted from one of the laser chips of thelaser diode 1 is used for a “DVD”, while the diffracted light beam ofthe 1st degree obtained from the diffraction element 6 for the lightbeam emitted from the other laser chip of the laser diode 1 is used fora “CD”. Then, both the light beam for a “DVD” and the light beam for a“CD” are focused to a same focal point on the light receiving section 7a of the photodetector 7.

The above described optical system is advantageous in that thediffraction angle of the diffraction element 6 is small and thedisplacement of the spot of light on the light receiving section 7 a dueto the fluctuations of the ambient temperature is small. However, itshould be noted that unnecessary diffracted light can easily get to thelight receiving section 7 a to degrade the performance of the opticalsystem because of the small diffraction angle of the diffraction element6.

FIG. 21 is a schematic lateral view of the optical the optical system ofstill another embodiment of optical pickup device according to theinvention, comprising a two-wavelength type monolithic laser diode and adiffraction element 6 arranged on the forward light path. With thisarrangement, the light beam emitted from the laser diode 1 is made tostrike either a “DVD” or a “CD” after passing through the diffractiongrating 2, the diffraction element 6, the beam splitter 3, thecollimator lens 4 and the objective lens 5.

With this optical system, the diffraction angle of the diffractionelement 6 is defined in such a way that the optical axis of the lightbeam of the 0-th degree from the diffraction element 6 obtained for thelight beam emitted from one of the laser chips of the laser diode 1 andthat of the light beam of the 1st degree from the diffraction element 6obtained for the light beam emitted from the other laser chip of thelaser diode 1 agree with each other Thus, the light beams that are madeto strike the objective lens 5 along the same optical axis. Then, thelight beams coming out of the objective lens 5 are reflected by therespective optical discs and proceed all the way to the photodetector 7also along the same optical axis.

This optical system is advantageous relative to the optical system wherethe diffraction element 6 is arranged on the backward light pass fromthe optical discs in that it is more free from mutual displacement ofthe focal points and the two light beams can be focused accurately to asame spot on the light receiving section of the photodetector to improvethe performance of the optical pickup device.

However, it should be noted that, if the light emitting spots of thelaser diode and the diffraction grating are arranged close to each otherand the diffraction angle of the diffraction element 6 is made large,the intensity distribution of the spot of light on the “CD” becomeshighly asymmetric so that consequently it will be impossible to select alarge diffraction angle and hence unnecessary diffracted light can enterthe optical system to a large extent.

When the light receiving section of the photodetector is shared by the“DVD” and the “CD” while the light receiving section is divided intofour light receiving areas that are arranged radially as viewed from thecenter thereof so that a so-called astigmatism method is used fordetecting a focusing error signal and/or a so-called DPD method is usedfor detecting a tracking error signal, care should be taken about anumber of possible phenomena including that the efficiency ofutilization of light can be reduced due to an excessive low diffractionefficiency of the diffraction element, that the degree of freedom forregulating the optical system can be reduced due to an excessive smalldiffraction angle, that the optical system can show a degradedresistance to temperature changes due to the temperature dependency ofthe performance of the diffraction element and that the opticalperformance of the optical system can become degraded as a result of aninclined optical axis.

An optical disc device according to the invention comprises an opticalpickup device according to the invention and a rotary operatingmechanism for holding and driving an optical disc, or an opticalrecording medium, which may be a “DVD” or a “CD”. The rotary operatingmechanism is adapted to align the optical disc to itself by referring tothe chucking hole arranged at the center of the optical disc and holdthe optical disc at a peripheral area of the chucking hole to drive itto rotate.

The optical pickup device is supported in the optical disc device insuch a way that it is movable in radial directions of the optical discwith the objective lens disposed vis-a-vis the signal recording surfaceof the optical disc that is driven to rotate by the rotary operatingmechanism. Then, the optical pickup device is driven to move in a radialdirection of the optical disc by means of a feed mechanism.

The optical disc device further comprises a control circuit forcontrolling the optical pickup device, the rotary operating mechanismand the feed mechanism. The optical disc device additionally comprises ademodulation circuit for demodulating the signal read from the opticaldisc by the optical pickup device and output from the latter.

1. An optical pickup device comprising: a first light source foremitting a first light beam having a first wavelength; a second lightsource for emitting a second light beam having a second wavelengthdifferent from the first wavelength; an objective lens for focusing saidfirst light beam or said second light beam to the signal recordingsurface of an optical recording medium of a first matching to the firstwavelength of that of an optical recording medium of a second typematching to the second wavelength, whichever appropriate; aphotodetector for detecting the light beam focused on the signalrecording surface of the optical recording medium of the first type orthat of the optical recording medium of the second type, whicheverappropriate, by the objective lens and reflected by the signal recordingsurface; and a diffraction element having a pair of plates arranged onthe light path from the signal recording surfaces of the two pieces ofoptical recording medium to the photodetector, each of said platescarrying a diffraction grating formed on one of the surface planes,wherein a first plate of the pair of plates has a first diffractionangle and a second plate of the pair of plates has a second diffractionangle, and the first and second plates are mounted so that each plate isindependently adjustable along the direction of an optical axis of theoptical pickup device to regulate at least one of the first and seconddiffraction angles; at least either the first light beam adapted to beused for reading information signals from the signal recording surfaceof the optical recording medium of the first type and reflected by thesignal recording surface of the optical recording medium of the firsttype or the second light beam adapted to be used for reading informationsignals from the signal recording surface of the optical recordingmedium of the second type and reflected by the signal recording surfaceof the optical recording medium of the second type and being diffractedby the diffraction element, wherein the first reflected light beam andthe second reflected light beam being focused to a same spot on thelight receiving surface of the photodetector.
 2. An optical pickupdevice comprising: a first light source for emitting a first light beamhaving a first wavelength; a second light source for emitting a secondlight beam having a second wavelength different from the firstwavelength; an objective lens for focusing said first light beam or saidsecond light beam to the signal recording surface of an opticalrecording medium of a first type matching to the first wavelength orthat of an optical recording medium of a second type matching to thesecond wavelength, whichever appropriate; a photodetector for detectingthe light beam focused on the signal recording surface of the opticalrecording medium of the first type or that of the optical recordingmedium of the second type, whichever appropriate, by the objective lensand reflected by the signal recording surface; and a diffraction elementhaving a pair of plates arranged on the light path from the lightsources to the photodetector by way of the two pieces of opticalrecording medium, each of said plates carrying a diffraction gratingformed on one of the surface planes, wherein a first plate of the pairof plates has a first diffraction angle and a second plate of the pairof plates has a second diffraction angle, and the first and secondplates are mounted so that each plate is independently adjustable alongthe direction of an optical axis of the optical pickup device toregulate at least one of the first and second diffraction angles; eachof the first light beam adapted to be used for reading informationsignals from the signal recording surface of the optical recordingmedium of the first type and reflected by the signal recording surfaceof the optical recording medium of the first type and the second lightbeam adapted to be used for reading information signals from the signalrecording surface of the optical recording medium of the second type andreflected by the signal recording surface of the optical recordingmedium of the second type, and being diffracted by the diffractionelement, wherein the first reflected light beam and the second reflectedlight beam are focused to a same spot on the light receiving surface ofthe photodetector.
 3. An optical pickup device comprising: a first lightsource for emitting a first light beam having a first wavelength; asecond light source for emitting a second light beam having a secondwavelength different from the first wavelength; an objective lens forfocusing said first light beam or said second light beam to the signalrecording surface of an optical recording medium of a first typematching to the first wavelength or that of an optical recording mediumof a second type matching to the second wavelength, whicheverappropriate; a photodetector for detecting the light beam focused on thesignal recording surface of the optical recording medium of the firsttype or that of the optical recording medium of the second type,whichever appropriate, by the objective lens and reflected by the signalrecording surface; and a diffraction element having a pair of platesarranged on the light path from the light sources to the signalrecording surfaces of the two pieces of optical recording medium, eachof said plates carrying a diffraction grating formed on one of thesurface planes, wherein a first plate of the pair of plates has a firstdiffraction angle and a second plate of the pair of plates has a seconddiffraction angle, and the first and second plates are mounted so thateach plate is independently adjustable along the direction of an opticalaxis of the optical pickup device to regulate at least one of the firstand second diffraction angles; at least either the first light beamadapted to be used for reading information signals from the signalrecording surface of the optical recording medium of the first type andreflected by the signal recording surface of the optical recordingmedium of the first type or the second light beam adapted to be used forreading information signals from the signal recording surface of theoptical recording medium of the second type and reflected by the signalrecording surface of the optical recording medium of the second type,and being diffracted by the diffraction element, wherein the firstreflected light beam and the second reflected light beam being focusedto a same spot on the light receiving surface of the photodetector. 4.An optical disc device comprising: a rotary operating mechanism fordriving one or more than one optical discs operating so many pieces ofoptical recording medium as to rotate, and an optical pickup devicearranged opposite to the signal recording surfaces of the one or morethan one optical discs driven to rotate by said rotary operatingmechanism; said optical pickup device comprising: a first light sourcefor emitting a first light beam having a first wavelength; a secondlight source for emitting a second light beam having a second wavelengthdifferent from the first wavelength. an objective lens for focusing saidfirst light beam or said second light beam to the signal recordingsurface of an optical recording medium of a first type matching to thefirst wavelength or that of an optical recording medium of a second typematching to the second wavelength, whichever appropriate; aphotodetector for detecting the light beam focused on the signalrecording surface of the optical recording medium of the first type orthat of the optical recording medium of the second type, whicheverappropriate, by the objective lens and reflected by the signal recordingsurface; and a diffraction element having a pair of plates arrange onthen light path from the signal recording surfaces of the two pieces ofoptical recording medium to the photodetector, each of said platescarrying a diffraction grating formed on one of the surface planes,wherein a first plate of the pair of plates has a first diffractionangle and a second plate of the pair of plates has a second diffractionangle, and the first and second plates are mounted so that each plate isindependently adjustable along the direction of an optical axis of theoptical pickup device to regulate at least one of the first and seconddiffraction angles; at least either the first light beam adapted to beused for reading information signals from the signal recording surfaceof the optical recording medium of the first type and reflected by thesignal recording surface of the optical recording medium of the firsttype or the second light beam adapted to be used for reading informationsignals from the signal recording surface of the optical recordingmedium of the second type and reflected by the signal recording surfaceof the optical recording medium of the second type and being diffractedby the diffraction element, wherein the first reflected light beam andthe second reflected light beam being focused to a same spot on thelight receiving surface of the photodetector.
 5. An optical disc devicecomprising: a rotary operating mechanism for driving one or more thanone optical discs operating as so many pieces of optical recordingmedium to rotate; and an optical pickup device arranged opposite to thesignal recording surfaces of the one or more than one optical discsdriven to rotate by said rotary operating mechanism; said optical pickupdevice comprising: a first light source for emitting a first light beamhaving a first wavelength; a second light source for emitting a secondlight beam having a second wavelength different from the firstwavelength; an objective lens for focusing said first light beam or saidsecond light beam to the signal recording surface of an opticalrecording medium of a first type matching to the first wavelength orthat of an optical recording medium of a second type matching to thesecond wavelength, whichever appropriate; a photodetector for detectingthe light beam focused on the signal recording surface of the opticalrecording medium of the first type or that of the optical recordingmedium of the second type, whichever appropriate, by the objective lensand reflected by the signal recording surface; and a diffraction elementhaving a pair of plates arranged on the light path from the lightsources to the photodetector by way of the two pieces of opticalrecording medium, each of said plates carrying a diffraction gratingformed on one of the surface planes, wherein a first plate of the pairof plates has a first diffraction angle and a second plate of the pairof plates has a second diffraction angle, and the first and secondplates are mounted so that each plate is independently adjustable alongthe direction of an optical axis of the optical pickup device toregulate at least one of the first and second diffraction angles; eachof the first light beam adapted to be used for reading informationsignals from the signal recording surface of the optical recordingmedium of the first type and reflected by the signal recording surfaceof the optical recording medium of the first type and the second lightbeam adapted to be used for reading information signals from the signalrecording surface of the optical recording medium of the second type andreflected by the signal recording surface of the optical recordingmedium of the second type and being diffracted by the diffractionelement, wherein the first reflected light beam and the second reflectedlight beam being focused to a same spot on the light receiving surfaceof the photodetector.
 6. An optical disc device comprising: a rotaryoperating mechanism for driving one or more than one optical discsoperating so many pieces of optical recording medium as to rotate; andan optical pickup device arranged opposite to the signal recordingsurfaces of the one or more than one optical discs driven to rotate bysaid rotary operating mechanism; said optical pickup device comprising:a first light source for emitting a first light beam having a firstwavelength; a second light source for emitting a second light beamhaving a second wavelength different from the first wavelength; anobjective lens for focusing said first light beam or said second lightbeam to the signal recording surface of an optical recording medium of afirst type matching to the first wavelength or that of an opticalrecording medium of a second type matching to the second wavelength,whichever appropriate; a photodetector for detecting the light beamfocused on the signal recording surface of the optical recording mediumof the first type or that of the optical recording medium of the secondtype, whichever appropriate, by the objective lens and reflected by thesignal recording surface; and a diffraction element having a pair ofplates arranged on the light path from the light sources to the signalrecording surfaces of the two pieces of optical recording medium, eachof said plates carrying a diffraction grating formed on one of thesurface planes, wherein a first plate of the pair of plates has a firstdiffraction angle and a second plate of the pair of plates has a seconddiffraction angle, and the first and second plates are mounted so thateach plate is independently adjustable along the direction of an opticalaxis of the optical pickup device to regulate at least one of the firstand second diffraction angles; at least either the first light beamadapted to be used for reading information signals from the signalrecording surface of the optical recording medium of the first type andreflected by the signal recording surface of the optical recordingmedium of the first type or the second light beam adapted to be used forreading information signals from the signal recording surface of theoptical recording medium of the second type and reflected by the signalrecording surface of the optical recording medium of the second type andbeing diffracted by the diffraction element, the first reflected lightbeam and the second reflected light beam being focused to a same spot onthe light receiving surface of the photodetector.